1. Field of the Invention
The present invention relates to a piezoelectric motor, and more particularly, to a piezoelectric motor for generating an elliptic motion by adding a longitudinal vibration to a bending-direction vibration.
2. Description of the Related Art
Recently, much attention has been paid to a new piezoelectric motor (piezoelectric ultrasonic motor) using a piezoelectric member as a substitute for an electromagnetic motor. In the piezoelectric motor, a piezoelectric vibrator generates a high-frequency vibration having an infinitesimal amplitude, and the infinitesimal amplitude is transmitted through contact friction between a frictional member attached to the piezoelectric vibrator and a slider (or rotor), so that the slider can perform an infinitesimal motion. In comparison with a conventional electromagnetic motor, the piezoelectric motor has many advantages such as an availability of miniaturization, a high resolution, and a low noise.
FIG. 1 is a schematic view illustrating a structure of a conventional piezoelectric motor.
As shown in FIG. 1, the piezoelectric motor typically includes a piezoelectric member 10 and a frictional member 30 attached to one side of the piezoelectric member 10.
The piezoelectric member 10 is formed by laminating a plurality of piezoelectric devices made of ceramics or the like. Internal electrodes are formed on surfaces of the piezoelectric devices, so that the piezoelectric member can be partitioned into a plurality of vibration portions. A pattern of internal electrodes may be formed on the surface of each piezoelectric device in various shapes according to a type of vibration, a vibration direction, the number of attached frictional members, and positions of the attached frictional members. Wire lines or external electrodes 21 and 22 which connect the internal electrodes are provided to the piezoelectric member so that two vibration portions 11 and 14 or 12 and 13 that are disposed in a diagonal direction are applied with an in-phase alternating voltage. As described above, the frictional member 30 made of ceramics or a supperhardened alloy is attached on the one side surface of the piezoelectric member 10 so as to transmit the vibration externally.
More specifically, referring to FIG. 1, the piezoelectric member 10 is partitioned into four vibration portions 11, 12, 13, and 14. The two vibration portions 11 and 14 or 12 and 13 that are disposed in the diagonal direction are applied with the in-phase alternating voltage through the wire lines or external electrodes 21 and 22.
When the voltage is applied, the piezoelectric member 10 generates two vibration modes, for example, a stretching vibration mode and a bending vibration mode. In the stretching vibration mode, the piezoelectric member 10 is stretchably deformed in the longitudinal direction thereof, and in the bending vibration mode, the piezoelectric member 10 is bendably deformed in the thickness direction thereof.
Since the two vibration modes are generated simultaneously, an elliptic motion occurs in the frictional member 30. The elliptic motion of the frictional member 30 is transmitted to the slider or the rotor, so that a linear motion of the slider or a rotational motion of the rotor can be obtained.
In the conventional piezoelectric motor, a change in mass caused from the frictional member attached to the piezoelectric member influences a driving frequency of the piezoelectric motor. Therefore, it is difficult to electrically drive and control the piezoelectric motor.
In general, as a size of the piezoelectric member is decreased, a resonance frequency is increased. For example, if the length of the piezoelectric member is reduced down to 5 mm, the resonance frequency is increased up to 300 kHz or more. In such a high frequency, an acceleration of 200,000 mm/s2 or more is exerted on the surface of the frictional member. Due to the high acceleration, the frictional member may be detached from the surface of the piezoelectric member. As an approach for preventing detachment of the frictional member from the surface of the piezoelectric member, there is proposed a method of increasing an amount of coated adhesive. However, the increase in the amount of coated adhesive causes a problem of a great change in the resonance frequency.
In addition, since the frictional member is attached on the surface of the piezoelectric member, the position of attachment of the frictional member is irregular, so that the resonance frequency may be greatly changed.